1. Field of the Invention
The present invention relates to a porous ferrite core material for an electrophotographic developer, used in a two-component electrophotographic developer used in apparatuses such as copiers and printers, a resin-filled ferrite carrier and an electrophotographic developer using the ferrite carrier.
2. Description of the Related Art
An electrophotographic development method is a method in which development is conducted by adhering the toner particles in a developer to the electrostatic latent image formed on a photoreceptor, and the developer used in such a method is classified into a two-component developer composed of toner particles and carrier particles and a one-component developer using only toner particles.
As a development method using a two-component developer, among such developers, composed of toner particles and carrier particles, previously a method such as a cascade method has been adopted, but currently a magnetic brush method using a magnet roll predominates.
In a two-component developer, the carrier particles serve as a carrying substance to form a toner image on the photoreceptor in such a way that the carrier particles are stirred together with the toner particles in a developer box filled with the developer to impart an intended charge to the toner particles, and further, convey the thus charged toner particles to the surface of the photoreceptor to form the toner image on the photoreceptor. The carrier particles remaining on a development roll which holds a magnet again return from the development roll to the developer box to be mixed and stirred with the fresh toner particles and to be repeatedly used for a predetermined period of time.
In contrast to a one-component developer, a two-component developer is such that the carrier particles are mixed and stirred with the toner particles, thus charge the toner particles, and further have a function to convey the toner particles, and a two-component developer is excellent in the controllability in designing developers. Accordingly, two-component developers are suitable for full-color development apparatuses required to offer high image quality and for high speed printing apparatuses required to be satisfactory in the reliability and durability in image maintenance.
In two-component developers used in the above-described manner, the image properties such as the image density, fogging, white spot, gradation and resolution are each required to exhibit a predetermined value from the initial stage, further these properties are required to be invariant and to be stably maintained during the endurance printing. For the purpose of stably maintaining these properties, the properties of the carrier particles contained in the two-component developers are required to be stable.
As the carrier particles which form two-component developers, there have hitherto been used various carriers such as iron powder carriers, ferrite carriers, resin-coated ferrite carriers and magnetic powder-dispersed resin carriers.
Recently office networking has been promoted, and the age of monofunctional copiers develops into the age of multifunctional copiers; the service system has also shifted from the age of the system such that a contracted service man conducts periodic maintenance inclusive of the replacement of the developer to the age of the maintenance-free system; thus, the market has further enhanced demand for further longer operating life of the developer.
Under such circumstances, for the purpose of reducing the carrier particle weight and extending the developer operating life, Japanese Patent Laid-Open No. 5-40367 and the like have proposed a variety of magnetic powder-dispersed carriers in each of which magnetic fine particles are dispersed in a resin.
Such magnetic powder-dispersed carriers can be reduced in true density by decreasing the amounts of the magnetic fine particles and can be alleviated in stress caused by stirring, and hence can be prevented from the abrasion and exfoliation of the coating film and accordingly can offer stable image properties over a long period of time.
However, the magnetic powder-dispersed carrier is high in carrier resistance because the magnetic fine particles are covered with a binder resin. Consequently, the magnetic powder-dispersed carrier offers a problem that a sufficient image density is hardly obtained.
The magnetic powder-dispersed carrier is prepared by agglomerating magnetic fine particles with a binder resin, and hence offers, as the case may be, a problem that the magnetic fine particles are detached due to the stirring stress or the impact in the developing device or a problem that the carrier particles themselves are cracked probably because the magnetic powder-dispersed carriers are inferior in mechanical strength to the iron powder carriers and ferrite carriers having hitherto been used. The detached magnetic fine particles and the cracked carrier particles adhere to the photoreceptor to cause image defects as the case may be.
Additionally, although the magnetic powder-dispersed carriers can be produced by two methods, namely, a pulverizing method and a polymerizing method, the pulverizing method is poor in yield, and the polymerizing method involves complicated production steps, and hence both methods suffer from a problem that the production cost is high.
As a substitute for the magnetic powder-dispersed carrier, there has been proposed a large number of resin-filled carriers in which the voids in a porous carrier core material are filled with a resin. For example, Japanese Patent Laid-Open No. 2006-337579 proposes a resin-filled carrier prepared by filling a resin in a ferrite core material having a porosity of 10 to 60%, and Japanese Patent Laid-Open No. 2007-57943 proposes a resin-filled carrier having a three-dimensional laminated structure. These Japanese Patent Laid-Open Nos. 2006-337579 and 2007-57943 disclose that: various methods are usable as the method for filling a resin in a core material for a resin-filled carrier; examples of such a method include a dry method, a spray drying method based on a fluidized bed, a rotary drying method and a dip-and-dry method using a universal stirrer or the like; and these methods are appropriately selected according to the core material and the resin to be used.
Japanese Patent Laid-Open No. 2007-57943 also discloses that: it is preferable to reduce the pressure inside the filling apparatus when resin filling is conducted; it is difficult to fill the interior of the voids with a resin under normal pressure or under pressurized condition; the reduction of the pressure inside the apparatus enables to efficiently and sufficiently fill with a resin the voids inside the particles to facilitate the formation of the three-dimensional laminated structure.
Further, Japanese Patent Laid-Open No. 2007-133100 describes a carrier obtained by impregnating a resin into a porous magnetic material and a carrier obtained by coating the surface of a core material with a large amount of a resin. It is stated that the true specific gravities of these carriers are light, and hence by using these carriers in a refill developer for a two-component development method in which development is conducted while a refill developer having a toner and a carrier is being fed to a developing device, with a superfluous fraction of the carrier in the developing device being discharged, where necessary, from the developing device, the superfluous fraction of the carrier can be smoothly discharged together with the toner.
The porous magnetic powders described in these Japanese Patent Laid-Open Nos. 2006-337579, 2007-57943 and 2007-133100 include examples in which the pore volume of the core material is examined on the basis of the BET specific surface area or the oil absorption amount. However, the BET specific surface area is a surface area in itself, and the value thereof does not directly determine the actual porosity. Although the oil absorption amount reflects the pore volume to some extent, the oil absorption simultaneously measures the space between the particles as can be seen from the measurement principles thereof and hence does not lead to the actual pore volume. In general, the space between the particles is larger than the actual pore volume in the particles, and hence the oil absorption is insufficient in accuracy as an index for the purpose of filling a resin without extreme excess or deficiency. Additionally, these Japanese Patent Laid-Open Nos. 2006-337579, 2007-57943 and 2007-133100 do not include any description on the size of the pores located on the ferrite surface and filled with a resin and on the distribution of the pore size, and consequently, when a resin is actually filled, the filled resin amount varies among the particles or an insufficient uniformity of the filled resin is resulted. Consequently, the particles insufficiently filled with the resin are low in strength, and when the carrier is used in an actual machine, the cracking of the carrier particles occurs and fine particles are generated from the carrier particles to offer a cause for image defects.
Japanese Patent Laid-Open No. 2007-218955 describes the inclusion of SiO2, Al2O3 and the like in manganese ferrite, and also describes the pore size, pore volume and the like of the particles of a core material. Specifically, Japanese Patent Laid-Open No. 2007-218955 discloses that: the provision of a carrier core material, at a stage of the carrier core material before the resin coating, with the durability enabling to maintain a high resistance under the conditions of high voltage application remarkably improves the maintenance of the high resistance at the time of the high voltage application when used as an electrophotographic developer, and enables to prevent the breakdown and the degradation of the image properties; additionally, with respect to the spent resistance, it is important to obtain a carrier core material by forming a porous magnetic powder having a specific pore distribution property and by subjecting the porous magnetic powder to a treatment for providing the powder with a high resistance.
However, in this Japanese Patent Laid-Open No. 2007-218955, a nonmagnetic component is required to be contained in a large amount to lead to a high probability of the occurrence of a low magnetization, and hence it is difficult to obtain a core material particle having an intended magnetization.
Japanese Patent Laid-Open No. 2005-314176 discloses a spherical ferrite particle containing Mg, Mn or the like, and having in the surface layer thereof one or more metal oxides selected from SiO2, Al2O3 and TiO2.
In Japanese Patent Laid-Open No. 2005-314176, the pore volume of the spherical ferrite particle is set at 0.05 ml/g or less, thus the pore volume is small, and no ferrite particle having a high magnetization and a high resistance is obtained. Additionally, the spherical ferrite particle is to be used for a resin-coated carrier, and hence does not acquire the advantage imparted to a resin-filled carrier.
In a carrier for an electrophotographic developer, the magnetization and the resistance are the important properties and the balance between the magnetization and the resistance is required.
For the purpose of establishing the balance between the magnetization and the resistance, ferrite carriers using heavy metals such as Cu, Zn and Ni, or Mn have been used.
Nowadays, the environmental regulations become strict, the use of heavy metals such as Ni, Cu and Zn has come to be avoided, and the use of the metals adaptable to the environmental regulations is demanded. Thus, the ferrite compositions used as the carrier core materials are changing over from the Cu—Zn ferrite and the Ni—Zn ferrite to Mn-based ferrites such as manganese ferrite and the Mn—Mg—Sr ferrite.
However, even Mn-based ferrites using Mn are becoming the objects of various legal regulations from the viewpoint of the environmental regulation. In addition to the restriction that the above-described various heavy metals are not contained, it is demanded that ferrites containing Mn in an as small as possible amount are used as carrier core materials.
Japanese Patent Laid-Open No. 2009-175666 describes a resin-filled carrier using a porous ferrite core material that has a specific pore volume and a specific pore size. However, as is clear from the examples in Japanese Patent Laid-Open No. 2009-175666, this porous ferrite core material contains Mn, and hence it is difficult to claim that the above-described environmental consideration is put into practice.
As shown in these conventional techniques, although heavy metals are not used and the Mn content is reduced to be as small as possible, the following have not yet been obtained: a porous ferrite core material for an electrophotographic developer which core material permits controlling the magnetization and the resistance over wide ranges, has a high charging property and is suitable for a resin-filled ferrite carrier; a resin-filled ferrite carrier which maintains the advantages of the conventional resin-filled carrier as well as the above-described features, and is small in the amount of aggregated particles; and an electrophotographic developer using this ferrite carrier.